Vapor compression heat pump system

ABSTRACT

A compression refrigeration system that includes a compressor, a heat rejector, expansion means and a heat absorber connected in a closed circulation circuit that may operate with supercritical high-side pressure.

FIELD OF INVENTION

The present invention relates to a method for the operation of acompression refrigeration system including a compressor, a heatrejector, an expansion unit and a heat absorber connected in a closedcirculation circuit that may operate with supercritical high-sidepressure, using carbon dioxide or a mixture containing carbon dioxide asthe refrigerant in the system.

BACKGROUND OF THE INVENTION

Conventional vapor compression systems reject heat by condensation ofthe refrigerant at subcritical pressure given by the saturation pressureat the given temperature. When using a refrigerant with low criticaltemperature, for instance CO₂, the pressure at heat rejection will besupercritical if the temperature of the heat sink is high, for instancehigher than the critical temperature of the refrigerant, in order toobtain efficient operation of the system. The cycle of operation willthen be transcritical, for instance as known from WO 90/07683.

WO 94/14016 and WO 97/27437 both describe a simple circuit for realizingsuch a system, in basis comprising a compressor, a heat rejector, anexpansion means and an evaporator connected in a closed circuit. CO₂ isthe preferred refrigerant for these systems.

EP-A-10 043 550 relates to a compression refrigeration system using CO₂where an attempt is made to improve the heat pump efficiency of thesystem by controlling the compressor suction gas superheat.

Heat rejection at super critical pressures will lead to a refrigeranttemperature glide. This can be applied to make efficient hot watersupply systems, e.g. known from U.S. Pat. No. 6,370,896 B1.

Ambient air is a cheap heat source which is available almost everywhere.Using ambient air as a heat source, vapor compression systems often havea simple design which is cost efficient. However, at high ambienttemperatures, the exit temperature of the compressor may become low, forinstance around 70° C. for a trans-critical CO₂ cycle. The desiredtemperature of tap water is often 60-90° C. The exit temperature of thecompressor can be increased by increasing the exit pressure, but it willlead to a system performance drop. Another drawback with increasingpressure is that components will be more costly due to higher designpressures.

Another drawback occurring at high ambient temperatures is thatsuperheating the compressor suction gas, which normally is provided byan internal heat exchanger (IHX), is not possible, as long asevaporation temperature is higher than the heat rejector refrigerantoutlet temperature. Hence, there is a risk of liquid entering thecompressor.

A strategy to solve these problems is to regulate the evaporationtemperature such that it is below the heat rejector refrigerant outlettemperature. This will make superheating the suction gas possible andalso increase the compressor discharge temperature for better hot waterproduction; however, the system energy efficiency will be poor sincesuction pressure will be lower than necessary.

U.S. Pat. No. 6,370,896 B1 presents a solution to these problems, byusing a part of the heat rejector to heat the compressor suction gas.The full flow on the high pressure side is heat exchanged with the fullflow on the low pressure side. This will ensure superheating ofcompressor suction gas, and thereby secure safe compressor operation;however, the system efficiency drops compared to a system whichcompresses saturated gas (if possible) and which operates with a higherexit pressure to achieve a sufficient compressor discharge temperature.

SUMMARY OF THE INVENTION

An object of the present invention is to make a simple, efficient systemthat avoids the aforementioned shortcomings and disadvantages.

The present invention relates to a compression refrigeration system,comprising at least a compressor, a heat rejector, an expansion unit anda heat absorber. By superheating the compressor suction gas temperature,the compressor exit temperature can be increased without increasing theexit pressure and hot water at desired temperatures can be produced. Byusing a split flow (or flow splitting arrangement) at an appropriatetemperature from the heat rejector, it is possible to superheat thecompressor suction gas, for instance using a counterflow heat exchanger.After heating the compressor suction gas, the split flow is expandeddirectly to the low pressure side of the system. In this way, the twoparts of the heat rejector will have different heating capacity perkilogram water flow due to a lower flow in the latter part. It is hencepossible to adapt a water heating temperature profile even closer to therefrigerant cooling temperature profile. Hot water can be produced witha lower high side pressure, and hence with a higher system efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described in the following by way ofexamples only and with reference to the drawings in which,

FIG. 1 illustrates a simple circuit for a vapor compression system.

FIG. 2 shows a temperature entropy diagram for carbon dioxide withexamples of operational cycles for hot water production.

FIG. 3 is a schematic diagram showing an example of a modified cycle toimprove system performance and operating range.

FIG. 4 a schematic diagram showing another example of a modified cycleto improve system performance and operating range.

FIG. 5 shows a temperature entropy diagram for carbon dioxide withexamples of temperature profiles for the heat rejector.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a conventional vapor compression system comprising acompressor 1, a heat rejector 2, an expansion means 3 and a heatabsorber 4 connected in a closed circulation system. When using, forinstance CO₂ as refrigerant, the high-side pressure will normally besupercritical in hot water supply systems in order to achieve efficienthot water generation in the heat rejector, as illustrated by circuit Ain FIG. 2. Desired tap water temperatures are often 60-90° C., and therefrigerant inlet temperature to the heat rejector 2, which is equal orlower than the compressor discharge temperature, has to be above thedesired hot water temperature.

Ambient air is often a favorable alternative as a heat source for heatpumps. Air is available almost everywhere, it is inexpensive, and theheat absorber system can be made simply and cost efficiently. However,at increasing ambient temperatures, the evaporation temperature willincrease and the compressor discharge temperature will drop if thecompressor discharge pressure is constant, see circuit B in FIG. 2. Insome instances, the compressor discharge temperature may drop belowdesired tap water temperature. Tap water production at a desiredtemperature will then be impossible without help from other heatsources.

One way to increase discharge temperature is to increase high sidepressure, see circuit C in FIG. 2. But this will cause a reduction ofsystem efficiency.

One way to superheat the suction gas is to use an Internal HeatExchanger (IHX) 5, see FIG. 3. But for instance when heating tap water,the refrigerant is cooled down close to net water temperature, typicallyaround 10° C., in the heat rejector (2). If the evaporation temperatureis above this temperature, the suction gas will be cooled down insteadof superheated, see FIG. 2. Liquid would then enter the compressor 1,causing severe problems. It is important to avoid using the IHX 5 whenthe evaporation temperature is equal or higher than the net watertemperature.

The present invention will secure a suction gas superheat irrespectiveof ambient temperature. When the evaporation temperature, or otherappropriate temperatures, reaches a predetermined level, a split streamfrom the heat rejector 2 at a suitable temperature, is carried to a heatexchanger, for instance a counterflow heat exchanger, for compressorsuction gas heating. The compressor discharge temperature will increase,and hot water may be produced at high system efficiency, see circuit Din FIG. 2. After heating the compressor suction gas, the spilt stream isexpanded directly down to the low pressure side.

EXAMPLE 1

One embodiment of the invention includes leading the split stream (e.g.,through a stream splitting arrangement) through an already existing IHX5. An arrangement for bypassing the main stream outside the IHX 5, andleading the split stream through the IHX 5, then has to be implemented.There are various configurations for this embodiment. One alternative isto use two three-way valves 6′ and 6″, as indicated in FIG. 3. One orboth of three-way valves may for instance be replaced by two stopvalves. The split stream is expanded directly to the low pressure sidethrough an orifice 7 downstream of the IHX 5. The orifice 7 may bereplaced by other expansion means, and valves may be installed upstreamand/or downstream of the expansion unit for closer flow control throughthe expansion unit 7.

EXAMPLE 2

Another embodiment includes installing a separate heat exchanger 8, forinstance a counterflow heat exchanger, for suction gas heating. This isillustrated in FIG. 4. When the evaporation temperature, or other usabletemperatures, reaches a predetermined level, a split stream (i.e., astream splitting arrangement) is carried through the suction gas heater8 (e.g., through a stream splitting arrangement) by opening the valve10. This valve may be installed anywhere on the split stream line. Thesplit stream is expanded directly to the low pressure side through anexpansion means, for instance an orifice 7 as indicated in FIG. 4. TheIHX 5 can be avoided either by an arrangement on the high pressure sideindicated be the three way valve 9′, or a equivalent arrangement on thelow pressure side as indicated by dotted lines in FIG. 4.

Suction gas superheat may be controlled by regulation of the spiltstream flow. This can for instance be performed by a metering valve inthe split stream line. Another option is to apply a thermal expansionvalve.

As explained above, the invention will improve the energy efficiency athigh heat source temperatures, indicated by circuit D in FIG. 2. Byapplying embodiments of the present invention the high side pressure maybe further reduced compared to conventional systems optimum pressure.This is illustrated in FIG. 5. The first part of the heat rejector 2′will have a higher heating capacity relative to the water flow, comparedto the latter part of the heat rejector 2″. The temperature profile forthe water heating will be even better adapted to the cooling profile ofthe refrigerant, see water heating profile b in FIG. 5. Applying aconventional system will lead to the water heating profile a. As can beseen from FIG. 5, a temperature pinch will occur in the heat rejector 2.High side pressure will then have to be increased. With the embodimentsof the present invention, it is possible to produce hot water at desiredtemperature with a lower high side pressure, leading to an even moreenergy efficient system.

1. A compression refrigeration system configured for use with arefrigerant containing carbon dioxide, the system comprising: acompressor; a heat rejector; a first expansion unit; a heat absorber;and a stream splitting arrangement extending from the heat rejector at ahigh pressure side thereof and including a second expansion unit;wherein the compressor, the heat rejector, the first expansion unit, theheat absorber and the stream splitting arrangement are connected in aclosed circulation circuit that is configured to operate withsupercritical high-side pressure; and wherein the stream splittingarrangement is configured to generate a split stream flow to controlsuperheating of compressor suction gas and further configured to expandthe split stream flow from the high pressure side of the heat rejectorthrough the second expansion unit directly to a low pressure side of theheat absorber after heating the compressor suction gas.
 2. A systemaccording to claim 1, further comprising: a heat source operablyconnected to the compression refrigeration system; and wherein thestream splitting arrangement is configured to increase the temperatureof the compressor suction gas when the temperature of the heat source isabove a predetermined level.
 3. A system according to claim 1, whereinthe stream splitting arrangement is configured to control superheatingof the compressor suction gas, such that it has a temperature that isequal to a discharge temperature of the compressor.
 4. A systemaccording to claim 1, wherein the stream splitting arrangement includesa metering valve configured to regulate the split stream flow to controlthe superheating of the compressor suction gas.
 5. A system according toclaim 1, wherein the stream splitting arrangement includes a counterflowheat exchanger configured to heat the compressor suction gas.
 6. Asystem according to claim 1, further comprising: a first heat exchangerpositioned on the high pressure side of the heat rejector.
 7. A methodfor the operation of a compression refrigeration system including aclosed circulation circuit configured to operate with supercriticalhigh-side pressure, the closed circulation circuit having a compressor,a heat rejecter, a first expansion unit, and a heat absorber, thecompression refrigeration system further including a stream splittingarrangement extending from the heat rejector at a high pressure sidethereof directly to a low pressure side of the heat absorber, andincluding a second expansion unit, wherein the compression refrigerationsystem is configured for use with a refrigerant containing carbondioxide, the method comprising: generating a split stream flow throughthe stream splitting arrangement; controlling superheating of compressorsuction gas via the split stream flow; and expanding the split streamflow through the second expansion unit after heating the compressorsuction gas.
 8. A method according to claim 7, wherein said controllingof the superheating of the compressor suction gas includes increasingthe temperature of the compressor suction gas when the temperature of aheat source is above a predetermined level.
 9. A method according toclaim 7, wherein said controlling of the superheating of the compressorsuction gas includes controlling the superheating of the compressorsuction gas to a temperature that is equal to a discharge temperature ofthe compressor.
 10. A method according to claim 7, wherein saidcontrolling of the superheating of the compressor suction gas includesregulating the split stream flow.
 11. A method according to claim 7,wherein said controlling of the superheating of the compressor suctiongas includes controlling the superheating of the compressor suction gasvia a counterflow heat exchanger.
 12. A method according to claim 7,wherein said controlling of the superheating of the compressor suctiongas includes controlling the superheating of the compressor suction gasvia a heat exchanger positioned on the high pressure side of the heatrejector.
 13. A system according to claim 6, further comprising: asecond heat exchanger positioned in the stream splitting arrangement.14. A system according to claim 6, wherein said first heat exchanger ispositioned in the stream splitting arrangement.